Tutor profile: Isabel N.
What is the hydrophobic effect?
This is a classic biochemistry question! First off, water loves to form hydrogen bonds with itself. When non-polar compounds enter water, the compounds disrupts the hydrogen-bonding network and forces the water molecules to re-position themselves around each non-polar compound. However, according to the Second Law of Thermodynamics (which states that spontaneous reactions lead to an increase in entropy, or disorder), this is unfavourable, since the cage-like structures that form around each non-polar compound are orderly structures and therefore decrease the entropy of the total system. According to the Second Law of Thermodynamics, a favourable process would lead to greater entropy, or greater disorder. To solve this issue, the non-polar compounds aggregate together, so that fewer water molecules are needed to surround them and form a cage-like structure around them. If there are fewer orderly cage-like structures in the system, then the system is able to minimize the amount of order in the system, which is more favourable than the first scenario. The hydrophobic effect refers to this tendency of non-polar molecules to aggregate together in polar solvents (e.g. water).
Subject: Basic Chemistry
How do you find the empirical formula of a compound using percent composition by mass?
1. Usually the question will give the percentage composition by mass of the elements composing the molecule (e.g., 70% iron and 30% oxygen) 2. The trick is to start with 100 grams of the compound! 3. Determine the number of grams of each element (e.g., if we start with 100 grams and there 70% of the mass is iron while 30% of the mass is oxygen, then there are 70 grams of iron and 30 grams of oxygen) 4. Divide the number of grams of each element by its molar mass to get the number of moles of each element. Remember the formula for moles: Moles = Mass (in grams) / Molar Mass 5. Divide the number of moles of each element by each other to get the ratio of the elements (e.g., the number of moles of iron divided by the number of moles of oxygen) 6. The ratio indicates how many atoms are present in each molecule of the substance.
How do peptide, steroid, and amino-acid derived hormones differ from each other, and what are some examples of each?
This is a classic question on the body's endocrine system that comes up a lot on biology tests! First off, one way to classify hormones - signalling molecules that are secreted directly into the bloodstream and travel to distant target tissues - is by their structure. There are three main classifications of hormones: peptide hormones, steroid hormones, and amino-acid derived hormones. Peptide hormones are composed of polypeptide chains, chains of amino acids linked by peptide bonds. Since peptides are polar, peptide hormones cannot pass through the plasma membrane, and they must bind to extracellular receptors, where they trigger the transmission of a second messenger (such as cAMP). Peptide hormones usually have a rapid onset but their effects tend to be short-lived. One of the most well-known peptide hormones is insulin, a hormone that is released after every meal which allows glucose to enter into cells. Insulin has a fast onset but is relatively short-acting. Steroid hormones, on the other hand, are generally derived from cholesterol and are non-polar, meaning that unlike peptide hormones, they can pass through the plasma membrane (remember that the plasma membrane of cells is non-polar!). The receptors for steroid hormones are usually intracellular (inside of cells, in the cytosol) or intranuclear (inside of the nucleus) rather than extracellular. Additionally, steroid hormones usually have a slower onset than steroid hormones, but their effects last longer since their effects directly impact the DNA of cells. Estrogen and testosterone - hormones that promote sexual maturation - are two examples of well-known steroid hormones. They have a slower onset but their effects last longer than that of peptide hormones. Amino acid-derived hormones are, as their name suggests, derived from modified amino acids such as tyrosine and tryptophan. Thyroid hormones, for example, are derived from tyrosine with the addition of several iodine atoms. The interesting thing about amino-acid derived hormones is that some amino-acid derived hormones share features with peptide hormones, while others are more similar to steroid hormones. Thyroid hormones are similar to steroid hormones in that they bind to receptors intracellularly, and have a slower onest/longer duration than peptide hormones.
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